The use of blocking agents for temporary protection of isocyanate groups has been known for a long time. Blocked polyisocyanates are employed, inter alia, for the preparation of thermosetting one-component polyurethane stoving systems (1C PU stoving systems) which are stable to storage at room temperature. The blocked polyisocyanates are mixed here e.g. with polyesters containing hydroxyl groups, polyacrylates, other polymers and further constituents of lacquers and paints, such as pigments, co-solvents or additives. Self-crosslinking stoving systems which contain, as binders, polymers which contain both blocked isocyanates and hydroxyl groups in one molecule are another form of stoving lacquers which are stable to storage at room temperature.
Overviews of the use of blocked polyisocyanates are to be found, for example, in Wicks, Z. Progress in Organic Coatings 3 (1975) 73–99, Wicks, Z. Progress in Organic Coatings 9 (1981) 3–28, D. A. Wicks and Z. W. Wicks, Progress in Organic Coatings, (1999), 148–172.
The most important compounds which are employed for blocking polyisocyanates are ε-caprolactam, methyl ethyl ketoxime (butanone oxime), malonic acid diethyl ester, secondary amines and triazole and pyrazole derivatives, such as are described e.g. in EP-A 0 576 952, EP-A 0 566 953, EP-A 0 159 117, U.S. Pat. No. 4,482,721, WO 97/12924 or EP-A 0 744 423.
Secondary amines are described as blocking agents in EP-A 0 096 210. However, only amines containing alkyl, cycloalkyl and aralkyl groups are mentioned expressly as blocking agents there. Amines which contain functional groups with carbon-heteroatom multiple bonds or heteroatom-heteroatom multiple bonds are not mentioned explicitly there. The polyisocyanates blocked with the secondary amines described in EP-A 0 096 210 furthermore cannot be used generally for the preparation of solvent-containing or “high solids” lacquers since they have an inadequate solubility in organic solvents.
The most frequently employed blocking agents for isocyanates are ε-caprolactam and butanone oxime. While as a rule stoving temperatures of about 160° C. or higher are used in the case of ε-caprolactam, blocked 1C PU stoving lacquers in which butanone oxime has been employed as the blocking agent can already be stoved at temperatures 10 to 20° C. lower. For modern lacquer systems, however, a further lowering of the stoving temperatures required is aimed for, on the one hand in order to save energy and to be able to operate the stoving ovens at lower temperatures, and on the other hand in order also to be able to coat heat-sensitive substrates with such 1C PU lacquers.
Blocked polyisocyanates with which 1C stoving systems with lower stoving temperatures can be formulated can be prepared by blocking polyisocyanates with malonic acid diethyl ester, 1,2,4-triazole and diisopropylamine. However, these have the disadvantage that they are not sufficiently soluble in organic solvents, in particular in combination with polyisocyanates based on linear aliphatic diisocyanates, and crystallize out of these solutions and therefore cannot be employed generally. Furthermore, low-stoving 1C PU stoving systems based on polyisocyanates blocked with 3,5-dimethylpyrazole can be formulated. These have the disadvantage that 3,5-dimethylpyrazole is a solid at room temperature and is therefore difficult to meter. For metering in liquid form, it must be dissolved in a lacquer solvent at elevated temperatures or melted, which represents an additional working step. EP-A 0 0713 871 indeed discloses a process for the preparation of polyisocyanates blocked with 3,5-dimethylpyrazole in which 3,5-dimethylpyrazole is prepared in a lacquer solvent and the reaction solution is employed for the blocking immediately, without isolation of a solid, but an increased outlay on apparatus and/or time and the use of carcinogenic hydrazine for the preparation of the blocking agent are necessary in this process. Moreover, it cannot always be ruled out that by-products of the preparation of 3,5-dimethylpyrazole will get into the product and lead to impairments of the product, e.g. of its colour.
The present invention was therefore based on the object of providing blocked polyisocyanates which do not have the abovementioned disadvantages of the prior art and in particular render possible a lower crosslinking or stoving temperature than butanone oxime-blocked polyisocyanates, are stable to crystallization in organic solutions and can be prepared in a simple manner.